This paper presents preview of the new book titled “Security of Computer Systems and Networks” (original in Serbian language “Sigurnost računarskih sistema i mreža”) – textbook for University lectures, but also a book that covers majority of important areas concerning current state of security. The book is the result of lecturing experience, research in this area, and also practical experience that authors have in many fields, including but not limited to architecture and design of security products and software, network projects, consultancy, analysis of issues and problems and providing solutions to them. It is not just a textbook for students, as it is suitable for all IT professionals and also for company management including IT as well as general management for companies, organizations and government agencies that base their vital infrastructure on computer and mobile technology. It is suitable for decision makers and for people who want to get almost complete overview of the computer and network security as it is now. In some areas of computer security, such as intrusion prevention and detection systems, e-commerce, and proper network and operating system security administration, this book goes one step further, presenting some novelties in the field and suggesting new solutions for actual problems.